The present invention relates to an automatic transmission for use in a vehicle, and more particularly relates to a hydraulic fluid pressure control system for controlling the engagement of the gear speed stages of such a transmission.
Various automatic transmissions for use in vehicles are well known. Such an automatic transmission generally comprises a gear transmission mechanism which comprises a plurality of friction engaging mechanisms, and one or another of a plurality of gear speed stages is achieved by selectively providing operating fluid pressures to various ones of the friction engaging mechanisms. Further, various hydraulic fluid pressure control systems are well known for controlling said selective supply of operating fluid pressures to the friction engaging mechanisms. Such a hydraulic fluid pressure control system generally comprises: a source of hydraulic fluid pressure such as a pump; a line fluid pressure control valve which modifies the fluid pressure supplied from the fluid pressure source and which produces a line fluid pressure which is regulated to an appropriate line fluid pressure level; a throttle fluid pressure control valve which modifies the line fluid pressure and which produces a throttle fluid pressure which increases in accordance with increase of a parameter representative of the load upon the internal combustion engine of the vehicle--in most cases, in accordance with increase of throttle opening of the engine, which is taken as indicative of engine load; a governor fluid pressure control valve, which modifies the line fluid pressure and which produces a governor fluid pressure which increases in accordance with increase of the road speed of the vehicle; a manual selector valve which is controlled by hand by the driver of the vehicle and which is selectable between a plurality of positions which correspond to desired ranges of gear speed stages, typically including a "D" range and at least one forward range which is not a "D" range; and a plurality of speed shift valves, which selectively shift supply of actuating fluid pressures to the plurality of friction engaging mechanisms, according to various equilibrium relationships between the throttle fluid pressure and the governor fluid pressure, in a per se well known fashion.
Thus, in such an automatic transmission, the desired transmission speed range is manually set by the driver on the aforesaid manual selector valve, and then, within this speed range, the various speed shift valves corresponding to speeds which are allowed to be selected within this speed range are shifted to and fro according to the aforesaid balance relationships between the opposing governor fluid pressure and throttle fluid pressure, in accordance with the operating conditions of the vehicle and of the internal combustion engine of the vehicle, and thus the gear transmission mechanism is set to a particular speed stage.
Further, it is known for the aforesaid level of the line fluid pressure maintained by the line fluid pressure regulation valve to be increased in accordance with increase of the value of the throttle fluid pressure. This is usually done because when the vehicle is being operated at high engine load, i.e. high throttle opening, then the torque produced by the engine is high, and accordingly it is desirable to engage the friction engaging mechanisms of the automatic transmission (which are typically engaged by selectively switched supply of said line fluid pressure) more quickly and more tightly than when the engine is producing low torque. Accordingly, therefore, the value of the line fluid pressure provided by the line fluid pressure regulation valve is increased when the load on the internal combustion engine of the vehicle is high, so that the engagement of the friction engaging mechanisms may be better able to cope with the increased torque at this time.
A problem has arisen with prior art hydraulic fluid pressure control systems for automatic transmissions, in that, when the vehicle comprising the transmission is being driven, for example, at a high road speed with the manual range selection valve of the automatic transmission set to the "D" range, and then the manual range selection valve is manually shifted down by the driver of the vehicle to, for example, the "2" range, with the intention of obtaining strong engine braking for decelerating the vehicle, and when in accordance with this intention the throttle pedal of the vehicle is also released so that the throttle fluid pressure becomes very low or minimum, and thereby also, according to the above described operation, the line fluid pressure becomes minimum, then, although the vehicle's actual road speed still is high, the gear transmission mechanism may be directly shifted down to the second speed stage, and thereby the engine of the vehicle will be rotated at an unduly high rotational speed, i.e., will be over-revved, by the force of the momentum of the vehicle. This happens because, although the speed of the vehicle is still high, because the line fluid pressure has dropped to near minimum due to the aforesaid dropping of the throttle fluid pressure to very low or minimum, the governor fluid pressure control valve, which modifies this line fluid pressure to produce the governor fluid pressure which ought to be representative of the vehicle's road speed, is not able to produce a proper governor fluid pressure accurately representative of the vehicle's road speed, because such a proper governor fluid pressure would be higher than the actual current value of the line fluid pressure, because this line fluid pressure has dropped. Thus, because the governor fluid pressure control valve is of course unable actually to increase the value of the line fluid pressure to produce the governor fluid pressure, accordingly the governor fluid pressure has failed to be maintained at a level properly representing the true road speed of the vehicle. Accordingly, during engine braking, the downshift to, for example, the second speed stage has had a tendency to occur much earlier than it ought to have done.
The most serious possible consequent of this too early downshifting is that the engine of the vehicle is rotated by the momentum of the vehicle at such an excessively high rotational speed that it is seriously damaged. Even if this does not occur, severe shifting shock occurs at the time of this too early downshifting, and this is very uncomfortable for the occupants of the vehicle, and may possibly even cause a serious accident. Further, the durability of the friction engaging mechanisms in the gear transmission mechanism of the automatic transmission is deteriorated, because of the sudden repeated shocks to which they are subjected, and the amount of energy which they are required to absorb.